A model of edge-defined film-fed (EFG) crystal growth is developed to study melt growth of cylinders of the scintillator crystal cesium iodide (CsI). This system is characterized by strongly nonlinear interactions of heat transfer, capillarity, and die geometry that give rise to multiple solution states under a single set of operating conditions. A thermal-capillary instability is identified that stems from meniscus sag due to gravity, which causes an inflection point to appear in the meniscus shape. This shape allows two solutions to coexist that correspond to different crystal diameters and gap sizes (axial length of liquid bridge between die and crystal). Also identified is an instability of convective heat transfer characterized by a strong interaction of thermal convection with the geometry of the growth interface.
Cadmium germanium diarsenide (CdGeAs2,) crystals are very promising for infrared second harmonic generation. However, their use has been limited by optical absorption in the 5 pm region. The role of composition and dopants has been extensively studied, and some point defects have been identified which do affect transparency. While some low absorption material has been produced, it has not been reproducible and variations within boules have been a serious problem. In this paper, which reviews some recent work on this problem, we describe a surprising and complex correlation between optical transparency, dislocations and point defects.
CdGeAs2 is an important nonlinear optical infrared material. Room-temperature absorption and temperature-dependent photoluminescence (PL) of as-grown p-type bulk crystals and crystals doped with indium and tellurium have been measured. The intensity of an intervalence band absorption near 5.5 microns (0.225 eV) is correlated with the intensity of a PL band near 0.55 eV. Both of these optical features indicate the presence of a native shallow acceptor level at 120 meV above the top valence band. The 0.55-eV PL band is donor-acceptor-pair recombination between shallow donors and the shallow acceptor level. A second PL band peaking near 0.35 eV is donor-acceptor-pair recombination between shallow donors and a deeper acceptor at 300 meV above the top valence band. Doping with indium and tellurium produces n-type material. The intervalence band absorption at 5.5 microns is completely eliminated in the n-type samples. Indium donors are incorporated on the Cd site and Te donors are incorporated on the As site.
A considerable number of new crystalline compounds were prepared by reacting L-arginine (L-Arg) with a series of inorganic and organic acids. In addition to the usual arginine slats with composition L-Arg-HX, where HX represents various acids, an entirely new class of crystal with the composition L-Arg 2HX was discovered. Due to the presence of the optically active L-arginine component, all these crystal had non-centro-symmetric structures. Most of the crystal prepared in this study gave strong phase-matched second harmonic generation signals with a Nd:YAG laser, and some appear quite promising for nonlinear optical applications.
Based on the lithium vacancy defect structure model of non- stoichiometric lithium niobate, a correlation was found between the OH absorption band and dopant charge. In addition, the threshold concentration for suppressing the photorefractive effect in Sc doped lithium niobate was evaluated using phase-matching temperature measurements. It was also shown that hafnium can suppress the photorefractive effect. A criterion for searching for new nonphotorefractive dopants has been formulated. New possible candidates are suggested and crystals of some of them have been grown.
Ce doped and undoped SrxBa1-xNb2O6 (SBN) fibers grown by the laser heated pedestal growth (LHPG) technique in Stanford University were investigated by 2D scanning electron microprobe analysis. The SBN fibers grown along c  or a  axes often show radially distributed optical inhomogeneities (core effects) of varying magnitude. Ba enrichment and Sr reduction were primarily detected in the core which can be qualitatively described by a complex-segregation effect. This defect structure as a complex-congruency related phenomenon modified by the composition-control mechanism of LHPG system. Its radial dependence of effective segregation coefficient is described by the modified Burton-Prim- Slichter equation.
Compact and efficient tunable solid state UV sources are highly desirable for a variety of optoelectronic and medical applications. During the past decade single crystals with certain types of boron-oxygen anionic groups have been identified as providing excellent short wavelength transmission and nonlinear properties which allows them to efficiently convert longer wavelength laser light to the UV and vacuum UV regions. In this review the properties, crystal growth methods, defect issues and device applications for the most important of these materials are discussed.
Lithium niobate (LiNbO3) epitaxial thin films have been grown on c-plane sapphire and LiTaO3 (LT) substrates using solid-source MOCVD. The structural quality of LiNbO3 (LN) films grown on LT was found to be higher than for films grown on sapphire. Films on LT have a single in-plane orientation over a wide range of deposition temperatures (500 degree(s) to 750 degree(s)C), while growth on sapphire often leads to a small percentage of 60 degree(s) misoriented grains. The surface roughness of LN films on LT substrates is also much less than for films deposited on c-sapphire under similar conditions. While sapphire is a less suitable substrate for the deposition of optical quality films, it has provided an opportunity to study the effects of surface microstructure on film quality. Our findings are described. Prism- coupled, thin film analysis has revealed optical losses as low as 2 dB/cm (TMo mode) in 1000 angstroms thick LN films deposited on c-sapphire and 5 approximately 6 dB/cm (TEo mode) in 5000 angstroms films deposited on LT. Using LT, we have been able to deposit high crystalline quality LN films with very smooth surfaces below the Curie temperature of the substrate (600 degree(s)C), and etching techniques have revealed that the polarity of these films follows that of the substrate. This result suggests the possibility of depositing LN on periodically poled LT substrates for quasi-phase matched optical interactions.